Recently, a photopolymerizable adhesive, which can be polymerized by the irradiation with ultraviolet rays or visible light, is widely used in various fields such as glass processing, electronics, optoelectronics and optics instead of a thermosetting or solvent adhesive such as an epoxide resin adhesive, an acrylic resin adhesive or a synthetic rubber adhesive since the photopolyaerizable adhesive is polymerized in a few seconds to a few minutes and exhibits a practical adhesive property upon being irradiated with ultraviolet ray or visible light.
Conventionally, such photopolymerizable adhesive comprises a photopolymerizable monomer and a photoinitiator. The photopolymerizable monomer generally includes a mixture of a multifunctional (meth)acrylate having a relatively large molecular weight and a high viscosity such as a bisphenol A-modified epoxy dimethacrylate, a urethane-modified di(meth)acrylate or a polyester-modified di(meth)acrylate with a radically polymerizable monomer having a relatively small molecular weight and a low viscosity such as hydroxyethyl (meth)acrylate, cyclohexyl (meth)acrylate, isodecyl (meth) acrylate, dimethylaminoethyl (meth) acrylate, N-vinylpyrrolidone or N,N-dimethylacrylamide.
The photoinitiator generally includes benzoin, benzoinalkyl ethers, benzophenone, benzophenone derivatives, benzil, thioxanthone, acetophenones and Michler's ketone.
However, a conventional photopolymerizable adhesive generally has a poor ultraviolet transmittance at a wavelength of 400 nm or less. Consequently, such an adhesive is not suitable for bonding a lens, prism, beam splitter or detecting window of an optical instrument in which a high pressure mercury lamp, a metal halide lamp or a mercury xenon lamp is used as the light source since the light energy from said light source is largely lost due to the absorption by the adhesive layer and is not be efficiently used.
Further, the conventional adhesive layer is colored in yellow or brown resulting in deterioration of its ultraviolet transmittance or is crazed or cracked upon intensive exposure to ultraviolet ray after photopolymerization. Therefore, the conventional adhesive cannot be used in an optical system under ultraviolet rays and is impracticable even for indirect application such as bonding and assembly of an inspection window or reflector in an ultraviolet exposing device or of a protecting cover in an ultraviolet lamp.
In addition, the conventional photopolymerizable adhesive has so poor weather resistance that the adhesive layer is colored in yellow or is crazed upon exposure to the sun light after polymerization and therefore, the conventional photopolymerizable adhesive is also impractical for bonding a window glass or assembling a showcase for outdoor usage.
It is generally believed that photochemical reactions such as oxidation and/or degradation caused by the absorption of light such as ultraviolet rays cause coloring, discoloring or deterioration of the mechanical property of a resin composition such as an adhesive.
Previously, many methods for avoiding such coloring or discoloring were proposed. Such methods include employing compounds having no benzene ring as photopolymerizable monomers and using a specific compound as a photoinitiator.
Another method is the addition of ultraviolet absorbents such as benzotriazole derivatives Or benzophenone derivatives to a photopolymerizable adhesive.
All of the above methods are based on the investigation of principal photochemical reactions of the photopolymerizable monomers, photopolymerizable adhesives or the ultraviolet absorbent. However, no attention was paid to the product of the side reaction of the photopolymerizable monomer with the photoinitiator. Further, substantially no investigation was carried out on the photochemical reaction of the photopolymerizable monomer with the ultraviolet absorbent. Consequently, no photopolymerizable adhesive which can stand intense ultraviolet ray irradiation and long-time exposure to ultraviolet rays has not been obtained. Further, ultraviolet transmittance of a conventional photopolymerizable adhesive containing an ultraviolet absorbent was hardly gained after adhesion.